alpha-Adrenoceptors in brain can be studied readily by radioligand binding techniques. This provides valuable information not only on the distribution of receptors in brain regions, but also on the regulation of receptors. The usefulness of this technique is dependent in part on a radioligand with high specificity for the receptor under study. Our studies have shown that 3H-clonidine does not bind exclusively to alpha 2-adrenoceptor subtypes, but also interacts with alpha 1-adrenoceptors. In contrast, 3H-guanfacine labels a high affinity alpha 2 subtype with good selectivity, but 3H-lofexidine probably labels with both alpha 2 and alpha 1-adrenoceptor binding sites.